Filthy

All kinds of clothing (that includes accessories like necklaces etc.) can be filthy if worn by filthy creatures like zombies, fungi etc. Items can be filthy too and everything obtained by cutting filthy clothing will become filthy as well.
If you desire, you can disable this feature by adding the "no filthy clothes" mod (found under the balance tab in the world mods) while creating a world.

Washing filthy clothings

You can wash filthy clothing to get rid of all the penalties associated with it, to do so you will need three things:

• A cleansing agent; like soap or detergent, those can be found easily in houses and the amount you will need to use will vary based on the volume or the complexity of the item you want to clean.
• A lot of water; how much you need to clean an item is dictated by it's size and complexity, to satisfy such a request of water you will need to either use an unlimited source of water (like a river or a swamp) or, in case you don't have access to either, a large container of water will do.
• A cleansing tool; a washboard is the easiest and most accessible tool to clean filthy stuff; you need fabrication 1, one 2x4 and a cutting tool to build it, You can also install a washing machine into your vehicle, it will function the same way as the washboard while using less water.

To use them you need to activate the cleansing tool while having the clothes you want to clean and the cleansing agent in your inventory and the needed water in your proximity. You will be prompted to a cleansing interface where to choose the cleaning agent to use and wich clothes to clean, here you can also see how much water and cleansing agent you will need to use to clean the selected items. You can press enter after selecting the clothes you want to clean to begin the cleaning process.

Wearing filthy clothing

While not recommended, wearing filthy clothes can have its benefits if you don't have nothing else to wear or you need the extra storage space of a filthy backpack and you don't have the possibilities of cleaning it. You can't, however, wear a filthy piece of clothing if you have the squeamish trait.
Wearing a filthy clothing will give you a morale penalty based on the number of filthy clothes you are wearing and their coverage, this penalty will stack until reaching -46.
Beside the morale penalty, if you get hit in a part of body covered with the filthy piece of clothing you will have a really high possibility of getting infected, making wearing filthy clothes in combat particularly dangerous.